Embedded program hangs because UART transmission

Hi,

I have a problem with my embedded programming with TivaC launchpad. I have a lot of blocks that include I2C acquisition for an IMU, a UART for GPS and a UART for Xbee.

The last part of the program that I made was the Xbee transmission. Before I made the Xbee block all the program runs perfectly in my launch pad, but when I made the S-function for the serial transmission I found 2 problems.

First: When I begin the program all run fine for 5-6 seconds, but sometimes it can run for 2 minutes without a problem, but in some moment the program hangs.

Second: I have an ADC input from sonar, when I test and graph without the Xbee the signal are totally plain, but when I test with Xbee sending every 3 seconds the signal from the sonar measure with a pic every time the serial was sent.

I think that with this both errors the problem is the memory managing of the launchpad, but I can't find the way to test or solve it.

This is the code of the S-function for the xbee:

        /*
         * Include Files
         *
         */
        #if defined(MATLAB_MEX_FILE)
        #include "tmwtypes.h"
        #include "simstruc_types.h"
        #else
        #include "rtwtypes.h"
        #endif

        /* %%%-SFUNWIZ_wrapper_includes_Changes_BEGIN --- EDIT HERE TO _END */
        # ifndef MATLAB_MEX_FILE
        # include <Energia.h>
        # include <math.h>   
        # endif
        /* %%%-SFUNWIZ_wrapper_includes_Changes_END --- EDIT HERE TO _BEGIN */
        /*
         * Create external references here.  
         *
         */
        /* %%%-SFUNWIZ_wrapper_externs_Changes_BEGIN --- EDIT HERE TO _END */
        /* extern double func(double a); */
        /* %%%-SFUNWIZ_wrapper_externs_Changes_END --- EDIT HERE TO _BEGIN */

        /*
         * Output functions
         *
         */
        extern "C" void Radio_Link_Outputs_wrapper(const real_T *UART_1,
              const real_T *xD,
              const int_T u_width)
        {
        /* %%%-SFUNWIZ_wrapper_Outputs_Changes_BEGIN --- EDIT HERE TO _END */
        if (xD[0] == 1){
        # ifndef MATLAB_MEX_FILE  
        int as=0;    
        Serial4.print('$');
        for (as=0;as<u_width;as=as+1){  
        Serial4.print(UART_1[as]);
             if (as!=(u_width-1)){
                Serial4.print(',');
             }
        }
        Serial4.println('%');
        # endif   
        }
        /* %%%-SFUNWIZ_wrapper_Outputs_Changes_END --- EDIT HERE TO _BEGIN */
        }

        /*
          * Updates function
          *
          */
        extern "C" void Radio_Link_Update_wrapper(const real_T *UART_1,
              real_T *xD,
              const int_T u_width)
        {
          /* %%%-SFUNWIZ_wrapper_Update_Changes_BEGIN --- EDIT HERE TO _END */
        if (xD[0] != 1){
            # ifndef MATLAB_MEX_FILE
            Serial4.begin(57600);   
            # endif    
            xD[0] = 1;
        }
        /* %%%-SFUNWIZ_wrapper_Update_Changes_END --- EDIT HERE TO _BEGIN */
        }

The simulink diagram of the uart section:

Do you have some idea to solve it?

Thanks